That leads to question: What is supposed to have caused the big bang itself? What came before time and space existed? If space-time did not exist, how could everything appear from nothing? Explaining this still remains the most difficult problem of the big bang theory. Where did all the matter come from that the big bang displaced across the universe and what caused this huge explosion of energy...a mystery to me.

well, we could just use the same explanation that creationist (is that the right term?) use when asked where God came from.....it was always just there